A conventional contactless Flash memory array employs bit lines and source lines that are diffused into a silicon substrate. FIG. 1A conceptually illustrates the layout of part of a bank 100 in a conventional contactless Flash memory. In bank 100, n+ diffusion into a silicon substrate forms diffused bit lines 110 and diffused source (or ground) lines 120. Polysilicon floating gates 130 (poly 1) overlie channel regions, which are between diffused bit lines 110 and diffused source lines 120. Polysilicon word lines 140 (poly 2) cross over portions of diffused bit lines 110 and diffused source lines 120 that form the drains and sources of memory cells and also overlie associated floating gates 130.
FIG. 1B shows a cross section along a word line 140 in bank 100. As shown in FIG. 1B, channel regions 115 in the silicon substrate separate drain regions of diffused bit lines 110 from source regions of diffused source lines 120. Floating gates 130 overlie respective channel regions 115, with a gate insulator (e.g., gate oxide layer) between floating gates 130 and underlying channel regions 115. Word lines 140 overlie floating gates 130 with an insulating layer between each word line 140 and the underlying float gates 130 that are in a row corresponding to the word line.
Isolation structures 125 such as shallow trench isolation (STI) regions, conventional field oxide (LOCOS) regions, or heavily doped p+ field implant regions separate adjacent diffused lines 110 from each other. Similar isolation structures (not shown) also separate adjacent channel regions 115 from each other.
As shown in FIG. 1A, each diffused bit line 110 extends to an associated bank select cell 170. Bank select cells 170 include transistors between respective diffused bit lines 110 and contacts to respective metal bit lines 180 that are typically part of a first metal layer and that overlies corresponding diffused bit lines 110. Generally, each metal bit line 180 extends over a number of banks and is connected to a corresponding select cell in each of the banks. A bank select line 160 controls bank select cells 170 in the bank to determine whether diffused bit lines 110 in the bank are connected to respective metal bit lines 180.
Diffused source lines 120 extend to contact virtual ground devices (not shown) or to other structures for control of the voltages of diffused source lines 120 during erase, write, and read operations.
A memory cell 150 in bank 100 of FIG. 1A includes a single floating gate transistor, and the number of memory cells in bank 100 is equal to the number of floating gates 130. In operation, charge added to or removed from the floating gate 130 in a memory cell 150 during a program or erase operation changes the threshold voltage of the floating gate transistor in the memory cell 150. The binary value stored in a memory cell 150 depends on whether the memory cell 150 is in a state having a high threshold voltage or a low threshold voltage.
The layout of each memory cell 150 in the contactless Flash memory array of FIG. 1A includes areas for a floating gate transistor and surrounding isolation structures. To minimize the area of a memory cell, features in the memory cells have widths or lengths equal to the minimum feature size, f, permitted by the design rules governing manufacture of the Flash memory integrated circuit. Along the direction perpendicular to diffused bit lines 110, the features for a memory cell include a shared isolation structure 125, a drain region of diffused bit line 110, a channel region 115, and a shared source region of diffused source line 120 as illustrated in FIG. 1B. The width and length of each of these structures are at least as wide or long as the minimum feature size f. Accordingly, a memory cell has an overall length (counting half the size of the shared features 120 and 125) of about 3f. The overall width of the memory cell area includes the width of the associated word line 140 and half of the widths of two adjacent isolation structures or at least 2f. The minimum memory cell area is thus about 6f2 (3fxc3x972f).
The effective memory cell size for bank 100 must additionally include a pro-rata portion of integrated circuit area associated with required overhead circuitry such as bank select cells 170, source contacts (not shown), and any virtual ground structures. More specifically, in one architecture M cells on a diffused bit line 110 share one bank select cell 170, the effective size of each memory cell is greater than 6f2 by 1/M times the area of a bank select cell and any other overhead for the column.
A contactless memory architecture that reduces the required cell area could increase the storage density achieved in a Flash memory integrated circuit (IC) and could therefore decrease the cost of Flash memory ICs.
In accordance with an aspect of the invention, a contactless memory architecture has diffused lines that operate either as diffused bit lines or diffused source lines depending on the voltage through bank select cells to the diffused lines. Word lines crossing the diffused lines have underlying floating gates and channel regions between each pair of adjacent diffused lines, and no isolation structures are required between adjacent diffused lines. Elimination of these isolation structures significantly reduces the minimum cell area and permits a higher density of memory cells within a bank.
In the exemplary embodiment, each bank includes two sets of bank select cells, typically at opposite ends of the diffused lines. Each bank select cell except some corresponding to diffused lines at edges of a bank connect an associated metal line to a pair of diffused lines. One set of bank select cells connects the metal lines to pairs of diffused lines that are shifted relative to the pairs of diffused lines that the other set of bank select cells connect to the metal lines. For any pair of adjacent diffused lines, activating one set of the bank select cells connects both of the adjacent diffused lines to the same metal line, and activating the other set of the bank select cells connects the two adjacent diffused lines to different metal lines.
The metal lines act either as bit lines or as ground lines depending on which column of memory cells an access operation selects. The layout of the metal lines across a set of banks generally includes a zigzag pattern to accommodate the relative shift of the bank select cells at opposite ends of each bank, but other layouts are possible.
An access to a memory cell in a selected column biases metal lines to one side (e.g., the left) of a selected column of memory cells at a first voltage and biases metal lines on the other side (e.g., to the right) of the selected column of memory cells at a second voltage. The appropriate set of bank select cells is activated for the access to achieve a voltage difference between the pair of adjacent diffused lines associated with the selected column. The resulting drain/source voltage difference of the memory cells in the selected column permits access of the selected memory cell in the selected column, and the common voltages on all other pairs of adjacent diffused lines prevent access or significant disturbance of memory cells in unselected columns.
One embodiment of the invention is a bank of a memory such as a Flash memory. The bank includes memory cells arranged in rows and columns. Each memory cell includes a channel region in a substrate, a floating gate overlying the channel region, and a control gate overlying the floating gate. Diffused lines in the bank have portions that form source/drain regions of the memory cells, and every area of the substrate that is between an adjacent pair of the diffused lines contains the channel regions of memory cells that form a column in the bank.
The bank can additionally include first bank select cells and second bank select cells. Each first bank select cell includes a transistor between a corresponding adjacent pair of the diffused lines and a corresponding metal line. Each second bank select cell includes a transistor between a corresponding adjacent pair of the diffused lines and a corresponding one of the metal lines, but the adjacent pairs of diffused lines corresponding to the second bank select cells are offset relative to the adjacent pairs of diffused lines corresponding to the first bank select cells. Additionally, one first bank select cell may be connected to a single diffused line, e.g., the first diffused line in the bank, and one second bank select cell may be connected to a single diffused line, e.g., the last diffused line in the bank.
Another embodiment of the invention is a memory including multiple banks and metal lines connected to the banks. Each bank includes: diffused lines in a substrate; channel regions arranged in rows and columns; floating gates respectively overlying the channel regions; word lines respectively overlying rows of the channel regions; first bank select cells; and second bank select cells. In each bank, each area of the substrate that is between an adjacent pair of the diffused lines contains a column of channel regions. The first bank select cells that are coupled to pairs of the diffused lines as are the second bank select cells, but the pairs of diffused lines coupled to the second bank select cells are offset relative to the pairs of diffused lines coupled to the first bank select cells.
Metal lines connect to respective first bank select cells and respective second bank select cells in each bank. Generally, the memory further includes column decoding circuitry coupled to bias the metal lines for an access of a selected memory cell. One embodiment of the invention includes thermometer-type column decoders in addition to conventional column decoders, drivers, sense amplifiers, and programming circuits. In response to a column address signal for the access, the thermometer-type column decoders bias all metal lines connected to diffused lines to a first side of the selected memory cell at a first voltage and biases all metal lines connected to diffused lines to a second side of the selected memory cell at a second voltage. Typically, the first voltage is ground, and the second voltage is positive and at a level required for a write or a read operation. The conventional column decoding circuits connect sense amplifier and programming circuits to the metal lines associated with the selected column.